The present invention relates to an adhesive composition suitable for use on a shaped part made of polyester, in particular a polyester film, the adhesive comprising a copolyester and optionally being combined with light-sensitive substances, fillers and pigments.
Shaped parts of linear polyesters, in particular polyethylene terephthalate, especially in the form of fibers, threads or films, have gained great importance. For example, polyester films are used as base materials for photographic films, drafting films or duplicating materials. After orienting and setting, they are distinguished by high mechanical strength, good chemical resistance and dimensional stability, which, in addition to other properties, explains their economic significance.
In the manufacture of coated polyester films, a problem arises in obtaining good adhesion between the support film and the layers to be applied, while at the same time being able to wind and unwind the coated materials without blocking occuring.
In diazo printing, for example, alkaline developing is necessary for the production of images after exposure under an original with a suitable UV-lamp in order to produce the diazo dyestuff in the areas which have not been exposed. The films are thus subjected to the action of water or water vapor and elevated temperatures in an alkaline medium. It is particularly important here for there to be a flawless, strong bond between the polyester film and the applied layer or layers.
The anchoring of polymeric materials on films, such as polyethylene terephthalate, presents great difficulties. If the layers additionally contain chemicals of the type present in photochemistry and diazo printing and are to be subjected to exposure and developing processes in which the adhesion strength of the layers is, according to experience, placed under great stress, the adhesive forces must be particularly strong in order to maintain the bond without any defects occurring. There has therefore been no lack of attempts to develop products which bring about an improved adhesion of the polyester film.
For example, German Pat. No. 971 996 (equivalent to British Pat. No. 700,957) discloses the use of highly polymeric polyesters as adhesion-promoting layers. It is also known (German Pat. No. 25 55 783, equivalent to U.S. Pat. No. 4,139,506) to employ, on polyester films, adhesive compositions which are composed of a mixture of copolyesters and polyisocyanate, optionally in combination with light-sensitive and other components, which contain an admixture of at least one cellulose ester, a copolymer of vinylidene chloride, a copolymer of methylvinyl ether and maleic acid anhydride or a copolymer of acrylic acid esters or methacrylic acid esters, or an admixture of hexamethoxymethyl melamine, the ratio of copolyester to admixture being in the range of 20:1 to 0.5:1.
When polyisocyanates are used as adhesive layer elements, however, either adhesive properties develop only after a certain storage period or the storability of the layers is reduced because they are sensitive to humidity or stick together. When the anchoring layers are heated to higher temperatures to counteract this disadvantage, they may turn yellow or cause incompatibility with the light-sensitive layer to be applied.
Copolyesters are themselves thermoplastic substances which under heat stress, as is customary in developing processes for copying materials, lose their adhesive power in an undesirable manner.
In connection with a two-component diazotype material comprising a support layer of polyethylene terephthalate and a binder-containing diazotype layer applied thereto, German Pat. No. 28 30 003 (equivalent to British Pat. No. 1,568,468) discloses the use of a binder composed of an adhesion-promoting copolymer of isophthalic acid and/or terephthalic acid, a glycol having 2 to 6 methylene groups and/or polyoxylakylene glycol.
However, it has been found that these layers cannot in all cases bear maximum stresses, such as occur, for example, in diazotype printing.